Synthetic calcium carbonate, referred to hereafter as precipitated calcium carbonate (PCC), is a known filler and coating pigment, particularly implemented in paper applications. It is obtained by hydration of calcium oxide (CaO), or “burnt lime”, forming a suspension of calcium hydroxide (Ca(OH)2); this hydration step is also referred to as a step of “slaking lime”. The obtained calcium hydroxide is thereafter precipitated by bubbling CO2 gas through the suspension, to form PCC.
Depending on the precipitation conditions, various polymorphs of PCC may be obtained, including aragonitic and scalenohedral polymorphs. The nature of the polymorph is generally determined based on analysis of scanning electron microscope (SEM) images of the product, aragonite crystal being generally needle-shaped as opposed to the ovoid form of scalenohedral products.
The skilled man refers to the PCC production capacity in terms of the weight of PCC produced relative to both the carbonation time and the weight of the final PCC suspension. Increasing this production capacity, namely by decreasing the carbonation time without overly increasing the final PCC suspension weight through addition of water to limit suspension viscosity, represents a goal of significant economical interest.
For the purpose of the present invention, carbonation time is the time from the start of introduction of CO2-containing gas to a calcium source in suspension to reaching a minimum suspension conductivity.
A number of studies have addressed decreasing carbonation time, including WO 01 07365, in which this is said to be achieved by reducing the carbonation reactor pressure, through which CO2 gas is bubbled, to below atmospheric pressure. However, this approach implies costly modifications to the PCC production equipment.
Additives are also known to be implemented during the PCC production process in order to reduce carbonation time. According to <<Change of formation yield and characterization of PCC particle synthesized with impurity ions by carbonation process>> (Materials Science Forum, 510-511, March 2006, pp. 1026-1029), this is achieved by addition of ions, such as aluminium, iron and magnesium; however, the crystallographic structure, or polymorph, of the resulting PCC differs relative to the PCC that would be obtained via a process excluding these ions.
In <<Morphological characteristics and aggregation of calcite crystals obtained by bubbling CO2 through Ca(OH)2 suspension in the presence of additives>> (Powder Technology, 130, 2003, pp. 307-315), the addition of citric acid, sucrose or calcium lignosulfonate to a suspension of slaked lime prior to the carbonation process is remarked to significantly extend carbonation time, whereas polyethylene glycol (of molecular weight 300 g/mol) is proposed to decrease this time. However, as shown in the Examples section herebelow, processes implementing polyethylene glycols fail to provide the improvement in PCC production capacity provided by the process of the present invention.
Lastly, WO 2005/000742 and WO 2004/106236 disclose a PCC production process in which a polyacrylate and polyphosphate are added prior to completion of the carbonation reaction. These documents fail to refer to any influence these additives might have on carbonation time. Indeed, as shown in the Examples herebelow, PCC production processes implementing such simple polyacrylates do not allow the same, advantageous improvement in PCC production capacity as do the processes of the present invention. Additionally, these documents clearly suggest that the crystallographic PCC structures obtained in the presence of the mentioned additives differ from that obtained in their absence.
As such, the skilled man has no leading line suggesting a process to solve the problem of improving the PCC production capacity, namely by reducing carbonation time, subject to the following constraints:                to identify a chemical additive allowing the carbonation time to be reduced without requiring extensive and often costly modifications to be made to his production equipment;        to identify a chemical additive that does not change the crystallographic structure of the obtained PCC relative to the structure obtained by the same process but in absence of said additive;        to identify a chemical additive that does not lead to overly high PCC suspension viscosities, requiring the addition of significant amounts of dilution water, said dilution water being a limiting factor in the PCC production rate;        to identify a chemical additive that leads to an aqueous suspension featuring a high PCC solids content, the solids being among the factors contributing to the overall production costs of the PCC material.        